ABCB1 genetic variants in leukemias: current insights into treatment outcomes

Abstract

Despite improvements in treatment of different types of leukemia, not all patients respond optimally for a particular treatment. Some treatments will work better for some, while being harmful or ineffective for others. This is due to genetic variation in the form of single-nucleotide polymorphisms (SNPs) that affect gene expression or function and cause inherited interindividual differences in the metabolism and disposition of drugs. Drug transporters are one of the determinants governing the pharmacokinetic profile of chemotherapeutic drugs. The ABCB1 transporter gene transports a wide range of drugs, including drugs used in leukemia treatment. Polymorphisms in the ABCB1 gene do affect intrinsic resistance and pharmacokinetics of several drugs used in leukemia treatment protocols and thereby affect the efficacy of treatment and event-free survival. This review focuses on the impact of three commonly occurring SNPs (1236C>T, 2677G>T/A, and 3435C>T) of ABCB1 on treatment response of various types of leukemia. From the literature available, some of the genotypes and haplotypes of these SNPs have been found to be potential determinants of interindividual variability in drug disposition and pharmacologic response in different types of leukemia. However, due to inconsistencies in the results observed across the studies, additional studies, considering novel genomic methodologies, comprehensive definition of clinical phenotypes, adequate sample size, and uniformity in all the confounding factors, are warranted.

Keywords:

• leukemia
• ABCB1 polymorphisms
• chemotherapy response
• survival

Introduction

Leukemia, a group of cancers that start in blood forming cells of the bone marrow, can affect anyone, including children. By considering whether leukemias are acute or chronic, and whether they are myeloid or lymphocytic in origin, they can be divided into four main types: acute myeloid leukemia (AML), chronic myeloid leukemia (CML), acute lymphoblastic leukemia (ALL), and chronic lymphocytic leukemia (CLL). Acute leukemias have many subtypes that vary in the response to treatment. Treatment of leukemias depends on the type of leukemia, certain features of the leukemia cells, the extent of the disease, prior history of treatment, as well as the age and health of the patient.^1,2 The genetic profile or specific characteristics of the leukemia cells as determined in the laboratory are used to determine the type of treatment that may be most appropriate. Most leukemia patients are treated with chemotherapy, while some may also have radiation therapy, biological therapy, targeted therapy, and bone marrow transplantation.^1,2

Treatment challenges in leukemias

For most cases of AML, treatment is usually chemotherapy, which is divided into two phases – remission induction (often called induction therapy) and consolidation (postremission therapy). Patients with different subtypes of AML can have different outlooks and response to treatment. After diagnosis, AML patients are categorized into three outcome risk profiles (favorable prognosis, intermediate risk profile, and poor outcome risk profile) and treated accordingly. However, for some unknown reasons, some respond well and some do not. Accordingly, adult AML patients have a 25% 5-year overall survival (OS) rate, whereas children and teens younger than 15 years have 66% OS rate.¹ New discoveries are needed to increase the chances of survival for AML patients who do not respond well to treatment.

The understanding of aberrant tyrosine kinase activity as the molecular mechanism behind CML development led to the development of tyrosine kinase inhibitor (TKI) drugs such as imatinib mesylate (IM).^3,4 The molecularly targeted drug IM has become the gold standard drug for the first line treatment of CML. An event-free survival (EFS) of 86% and OS of 88% for CML patients on IM therapy was reported by a 6-year update of the IRIS study.⁵ Furthermore, this resulted in overcoming the difficulties encountered with previous therapeutic approaches.^3,6 Despite these excellent results, approximately 30%–40% of CML patients on IM therapy develop resistance.⁷ In a significant proportion of CML patients, achievement of prolonged response to IM is still a daunting problem due to development of resistance or suboptimal response to IM.

Treatments for ALL patients include chemotherapy, biological agents, and transplant. Most chemotherapy plans for ALL involve three steps which are induction, consolidation, and maintenance. Chemotherapy and corticosteroids are in the treatment protocols. Different types of ALL patients may be treated differently. Younger adults with ALL have better long-term survival rates than older adults with ALL. Nevertheless, survival rate for childhood ALL have risen dramatically during the last 10–15 years and more than eight out of ten children are now cured.⁸ According to National Cancer Institute report, the 5-year survival rate for children and teens younger than 15 years is 92%, whereas the overall 5-year survival for ALL is 70%.¹

CLL is treated by chemotherapy, radiation therapy, therapy using monoclonal antibodies, or bone marrow transplantation. Majority of patients follow an indolent clinical course with no or delayed treatment need and with a prolonged survival, while others experience aggressive disease requiring early treatment followed by frequent relapses. However, during the last few decades, several new chemotherapeutics drugs are being tried, and some of these are promising in targeting CLL. For CLL, an OS rate of 84% has been achieved with modern treatments.¹ Some of the available treatments can often induce disease remission, but they are not able to reach the cure, and hence CLL remains an incurable disease in virtually all cases. So, in CLL, further research on individual factors that can benefit treatment regimens need to be undertaken.

Pharmacogenomics of drug response

Earlier, most patients with a specific type of leukemia were given the same treatment. The realization that some treatments worked better for some patients than for others led to research into the genetic differences seen in patients and in tumors. The completion of the Human Project in 2003 had provided increasingly comprehensive information on the genetic variations among individuals that are responsible for this variation in treatment response. Genetic variations in the form of single-nucleotide polymorphisms that affect gene expression or function in both normal and cancer cells can cause inherited interindividual differences in the metabolism and disposition of medications. Single-nucleotide polymorphisms (SNPs) between individuals influence how effective and safe a drug is for a person.

Pharmacogenetics incorporates information on how inherited genetic variation in a gene can affect a patient’s response to chemotherapeutic agents and aims to use this knowledge to tailor therapy for improved response and reduced toxicity. Meanwhile, pharmacogenomics is a broader strategy which elucidates the entirety of pharmacologically relevant genes, including the effects of genetic variation in single genes, the interaction among genes in biological and pharmacological pathways, the phenotype emerging from these variations, and the effect of the phenotype on drug response.⁹ Studies of inherited variability in drug targets or target pathways, variation in genes encoding drug-metabolizing enzymes or drug transporters, and genetic polymorphisms in genes encoding proteins, all of which indirectly influence drug response, comprise the field of pharmacogenomics. It also includes how these genetic variations interact to produce inherited drug-response phenotypes.^10,11 Variations in drug metabolizing enzymes, drug transporters, and drug targets are the most practical aspects of pharmacogenomics. The approach of personalized medicine, which uses predictive and prognostic biomarkers to direct patient management, holds great promise in improving the efficiency of treatment and outcomes of leukemia patients. In order to realize personalized medicine, there is the need to understand the molecular mechanisms underlying interindividual differences in drug response, including pharmacological effects and side effects.

Genetic variation and drug resistance

Development of resistance to chemotherapeutic drugs, which results in treatment failure, is a severe limitation of chemotherapy in leukemia patients. Factors that can influence plasma and intracellular levels of chemotherapeutic drugs may contribute to the development of resistance. Genetic polymorphisms in key genes encoding drug transporters and drug metabolizing and binding enzymes may influence the intracellular delivery and therefore the effectiveness and toxicity of chemotherapeutic drugs.^12,13 Genetic polymorphisms in the form of SNPs that affect gene expression or function in both normal and cancer cells can cause inherited interindividual differences in the metabolism and disposition of medications. Accordingly, genetic variations of candidate genes could affect expression of corresponding proteins and thus may bring about differences in response to chemotherapeutic drugs. Given that genetic differences between individuals or population can impact the efficacy of drugs, defining pharmacogenetic differences among patients is regarded as an important aspect which needs to be addressed in understanding the development of resistance to chemotherapeutic drugs in leukemia patients.

Accumulating evidence strongly suggests that drug transporters are one of the determinants governing the pharmacokinetic profile of chemotherapeutic drugs. Membrane transporters play an important role in acquired and de novo drug resistance. Based on the direction in which they transport, transporters are often classified as efflux, influx, or bidirectional. Drug resistance mechanism in anticancer therapy has been well established by drug transporter proteins. Expression of higher than normal levels of a transmembrane protein which serves as an energy dependent efflux pump causes a reduction in the amount of drug that accumulates within cancer cells and is considered a common mechanism of multidrug resistance in cancer cells.^14,15 Variation in drug transporters is one of the most clinically relevant pharmacogenomics aspect of some of the chemotherapeutic drugs for leukemias.

In this review, attention is focused on ABC transporters, and specifically on currently understood information on the impact of genetic variations in ABCB1 gene on leukemia treatment that may suggest better strategies in future for the use of current therapeutic agents.

ABC superfamily

The ATP-binding cassette (ABC) transporter is a protein superfamily whose members are characterized by two highly conserved ATP binding cassettes. In the human genome, 48 different members, forming eight different subfamilies (A–G) have been identified, based on sequence similarities.¹⁶ The ABC superfamily of proteins are involved in the transport of intrinsic and extrinsic molecules such as ions, sugars, glycans, phospholipids, amino acids, peptides, proteins, drugs, and toxins. Internalization of those substrates such as molecules and drugs occurs by active transport, which is dependent on the hydrolysis of ATP. All eukaryotic ABC proteins are efflux pumps.

ABCB1 protein

Within the ABC transporter superfamily, the subfamily B member 1 (ABCB1) appears to be most important in the human body, especially for the disposition of xenobiotics. ABCB1 transports a wide range of drugs and xenobiotics from the intra- to extracellular compartment at many biological interfaces such as the intestine, liver, blood–brain barrier, and kidney. As a transporter, ABCB1 has a broad affinity spectrum for different anticancer agents such as docetaxel, paclitaxel, irinotecan, vincristine, doxorubicin, vinblastine, mitoxantrone, teniposide, topotecan, etoposide, imatinib, sunitinib, etc.^17–19

The ABCB1 gene which codes the ABCB1 protein is located on chromosome 7q21.12. It spans 28 exons in a genomic region spanning 209.6 kb²⁰ and is one of the 49 putative members in the superfamily of human ABC transporters.²¹ The messenger RNA (mRNA) is 4,872 bp in length, includes the 5’ untranslated region (Ref seq accession NM_000 927.3), and can encode a protein of 1,280 amino acids in length, which is named the plasma membrane glycoprotein (P-glycoprotein [P-gp]) which is 170 kDa.²⁰ ABCB1 was formerly termed multidrug resistance gene (MDR1) since P-gp was observed to be overexpressed in tumor cells and led to the commonly known phenomenon of multidrug resistance against certain antineoplastic agents.²¹ Being involved in the extrusion of amphoteric compounds, it is also known as the Traffic ATPase.²² Recently, the common ABC transporter nomenclature has been applied in naming the gene and protein of P-glycoprotein as ABCB1 (in italics and nonitalics, respectively).

One of the main functions of ABCB1 include first- pass elimination of orally administered drugs to limit their bioavailability, by effluxing drugs from the lumen-facing epithelia of the small intestine and colon and from the bile-facing canaliculi of the liver. ABCB1 mediated drug disposition is influenced by modulation of ABCB1 gene expression and/or ABCB1 activity by various mechanisms. Overexpression of these transporters on plasma membranes cause increased efflux and decreased intracellular accumulation of many anticancer drugs, leading to multidrug resistance.²³

High levels of ABCB1 expression results in decreased intracellular concentration of drugs, and this will lead to development of cellular resistance to anticancer drugs. The expression level and functional integrity of ABCB1 may affect its pharmacogenetics and its interaction with drugs. Because of this, ABCB1 plays a significant role in drug efficacy and toxicity during treatment. Few studies have demonstrated that the level of ABCB1 activity determines the tissue distribution of drugs and affects the uptake from the gastrointestinal tract as well as elimination into urine or bile.¹⁶ Many substrates of ABCB1 have been well documented to be potent ABCB1 inhibitors, including channel blockers, calmodulin antagonists, immunosuppressants, and protein kinase inhibitors.²⁴

The discovery of ABCB1 provided a laboratory model that could explain the phenomenon of multidrug resistance. Cancer cells express ABCB1 proteins in different levels, thereby contributing to chemoresistance. For a better understanding of the significant variability in response to chemotherapeutics, polymorphisms in the ABC drug transporters have been extensively studied.

Genetic polymorphisms of ABCB1

Several publications have described polymorphisms of drug transporters as potential determinants of variability in drug disposition and efficacy. Polymorphisms in key drug transporter genes are known to influence intracellular drug delivery and, therefore, the effectiveness of drugs. With regard to ABCB1 gene also, a number of SNPs have been identified that are likely to have an effect on P-gp expression levels and function. SNPs in ABCB1 have been reported as modulators of ABCB1-mediated transport.

SNPs in ABCB1 have the potential to alter ABCB1 gene expression as well as P-gp function. Such SNPs are predicted to be associated with changes in both the pharmacokinetics and pharmacodynamics of several P-gp drug substrates, treatment response, as well as side effects.^25,26 According to NCBI SNP database, around 1,200 SNPs have been identified within the ABCB1 gene, of which 66 SNPs have been identified in the coding sequence so far, while more than 20 are known to be silent.²⁷ There is considerable heterogeneity in the literature and across the populations regarding the frequencies and association of this transporter gene polymorphisms with drug resistance. Among the various population groups, three SNPs 1236C>T, 2677G>T/A, and 3435C>T of ABCB1, which are located in exons 12, 21, and 26, respectively, are the most widely investigated for their clinical implications.^28–30

The SNPs C1236T, G2677T/A, and C3435T have been reported to be associated with altered mRNA expression levels, mRNA stability, and protein folding and influence drug pharmacokinetics.^31–33 However, another study found no association between these SNPs and ABCB1 gene function.³⁴ Changes in P-gp expression and function would be expected to alter the absorption, plasma concentration, tissue distribution, and excretion of its drug substrates.

The silent 3435C>T (rs1045642) was the first polymorphism of ABCB1 to be described and also modify P-gp expression.^35,36 This SNP modifies the gene expression of P-gp without altering the sequence of the protein (wobble mutation). However, it is probable that this SNP alters the mRNA stability,³⁷ as well as the folding of the protein, modifying its substrate specificity. The C3435T is the most widely investigated SNP of ABCB1. Differences in variant allelic frequency were observed among Caucasian, African, and Asian populations.³²

The tri-allelic nonsynonymous SNP 2677G>T/A (rs2032582) changes the serine to either threonine or alanine. This SNP has been reported to be associated with drug response and various diseases and also affects posttranslational modifications.³⁸ Biochemical analysis has confirmed that the wild-type G allele of 2677 alters drug transport by affecting drug-induced ATPase activity.³⁹ This SNP has been detected in various ethnic groups, and the highest variant genotype frequency was reported among Japanese (34%) compared to South African population, which presented only 2%.^40,41

ABCB1 1236T>C (rs1128503) encodes for the TM6 region, which is essential for substrate binding. This third most frequent SNP, 1236T>C, has also been reported to affect the expression and function of P-gp.²⁹ Highest frequency of 1236CC variant genotype was observed in German population, while lowest was among South Africans, where only 1% of variant CC genotype was detected.⁴¹

Genetic variants closely linked with other variants located on the same chromosome, known as haplotype, have also been documented to play an important role in drug response and disease susceptibility.⁴² This nonrandom association of SNPs is called linkage disequilibrium (LD). Genetic studies have identified a strong LD between SNPs in exons 12 (C1236T), 21 (G2677T), and 26 (C3435T).⁴³ Because of the significant LD, these SNPs are inherited to form two common haplotype patterns (T1236/T2677/T3435 or C1236/G2677/C3435).²⁹

Frequencies of these SNPs and haplotypes have been reported to vary across races and populations.³² Kroetz et al⁴⁴ reported 16 variants specific to African Americans, 8 to Caucasians, and 3 to Asian Americans. Despite the variant allele frequencies being higher in African Americans, the three SNPs were reported³⁷ to be twice as common in Caucasians as in African Americans. Therefore, ABCB1 substrates may be transported differently depending on racial and genetic background of individuals.

Owing to the important role in the drug disposition process, focus is given on the role of these SNPs as potential determinants of interindividual variability in drug disposition and pharmacologic response in different types of leukemia.

ABCB1 variants and AML treatment

For all forms of AML in adult patients, a combination of cytarabine and various doses of different anthracyclines had been the mainstay of treatment in the last four decades. Addition of an occasional third agent to this combination chemotherapy regimen has been found to be effective for treatment of some AML patients. However, it is far from ideal. The traditional ‘one size fits all’ regimen is not appropriate for AML. With the current forms of treatment, nearly 35%–40% of patients younger than 60 years of age are likely to achieve long-term survival. However, wide variation in outcome among genetically distinguishable subsets of the disease has been encountered. Some subtypes show notoriously poor outcome. Likewise, the overall prognosis remains highly unsatisfactory for patients who are more than 60 years of age. Poor prognosis, development of drug resistance, and death within 2 years of remission are common presentations in most adult AML patients.⁴⁵ This warrants the need for urgent therapeutic improvements in AML.

For a better understanding of the significant variability in response to chemotherapeutics, polymorphisms in the ABC drug transporters have been extensively investigated in AML patients. P-gp expression and activity profile, which are influenced by ABCB1 polymorphisms, importantly in exons 12 and 26, have been documented as factors that contribute to AML resistance to chemotherapy.^29,46 In AML patients, elevated P-gp expression and activity have been considered as adverse prognostic factors associated with refractory and relapsed disease.⁴⁷ Furthermore, dosage adjustment has been reported to be dependent on ABCB1 polymorphisms, which in turn has been implicated to be related to P-gp status.^35,48

In AML, complete remission rate and drug resistance are related to the function and expression of ABCB1.⁴⁹ The expression and functional drug efflux activity of ABCB1 was reported to increase with patient age, from 17% in patients less than 35 years old to 39% in patients aged 50 years or older.⁴⁹ Seedhouse et al⁵⁰ studied the expression and genetic polymorphisms of ABCB1 in 817 AML patients. These researchers observed that the 3435TT genotype (which results in unstable mRNA) of ABCB1 had a significant effect on P-gp expression. But this was observed only in 40% of cases in which mRNA and protein were detectable. According to Seedhouse et al,⁵⁰ low white blood cell count, secondary AML, and poor-risk cytogenetics had a much higher impact on prognosis than genetic polymorphisms of ABCB1 in AML blasts.

In addition, a strong link between ABCB1 genetic variants and P-gp expression with poor survival in AML patients has been reported.^51,52 Scheiner et al⁵¹ examined the relationship between ABCB1 polymorphism (C1236T, C3435T) and P-gp expression activity in 109 Brazilian AML patients to understand the possible relationship between these factors and their clinical significance. They reported achievement of better 5-year OS and 5-year EFS rates in patients presenting with genetic variant CC in exon 12, followed by those presenting the variant CT in exon 26. According to Scheiner et al,⁵¹ polymorphisms in the ABCB1 gene and the levels of P-gp expression could be useful to identify prognosis in AML patients.

Green et al⁵² and Falk et al⁵³ investigated the influence of SNPs 1199G>A, 1236C>T, 2677G>T/A, and 3435C>T in Swedish AML patients with de novo normal karyotype. Poorer OS was observed in patients with 1236 C/C or 2677 G/G genotypes than patients with other genotypes (p=0.03 and 0.02 respectively). Furthermore, leukemic cells from 1236 T/T and 2677 T/T patients demonstrated significantly higher susceptibility to mitoxantrone (p=0.02) and more susceptible to etoposide and daunorubicin (p=0.07–0.09) but not to cytarabine in in vitro studies.⁵² In the subgroup analysis⁵³ based on FLT3 and NPM1 status of the patients, FLT3 wild-type 1236 C/C patients had significantly shorter OS compared to patients carrying variant allele (median OS 20 vs 49 months respectively, p=0.017). Also, those patients with FLT3 wild-type 2677 G/G genotype showed an inferior outcome, compared to patients carrying the variant allele (median OS 20 vs 35 months respectively, p=0.039). These researchers concluded that ABCB1 1236C>T and 2677G>T might be used as prognostic markers to distinguish relatively high-risk patients in the intermediate-risk FLT3 wild-type group of AML patients as part of individualizing treatment strategies in future.

In 263 intermediate-risk Chinese AML patients treated with anthracycline and cytarabine, He et al⁵⁴ investigated the influence of polymorphisms G2677T/A, C1236T, and C3435T. This study reported that patients with TTT haplotype had a longer OS compared with those without TTT haplotype, and hence TTT haplotype was possibly related to the OS, EFS, and relapse in Chinese patients with AML. In a meta-analysis involving seven cohort studies with 1,241 AML patients undergoing standard chemotherapy (cytarabine plus anthracyclines), Megías-Vericat et al⁵⁵ reported significantly higher OS among carriers of the variant allele of 1236C>T, 2677G>T/A, and 3435C>T, with Caucasians showing consistent results in OS.

ABCB1 variants and ALL treatment

With the introduction of risk-directed therapy and current treatment protocols based on multiagent chemotherapy, the survival rates of ALL, especially of childhood ALL, has increased significantly. Nearly 85% of ALL patients achieve a long-term remission.⁵⁶ But still, nearly 15% of ALL children experience relapse due to substantial variation in treatment response.^57,58 There is still a group of patients for whom therapy fails, and some patients who experience severe toxicity.⁵⁹ Even though genomic alterations that are somatically acquired have long been recognized as hallmarks of ALL, inherited genetic variations (germ line) have also emerged as important determinants of interpatient variability in treatment response and toxicities of ALL patients. Increasing evidence that is emerging indicate that inherited genetic variations play significant roles in determining patient’s risk of relapse. Treatment failures depend on inherited SNPs in genes affecting drug metabolism, transport, and binding site affinity.⁶⁰

Treatment for childhood ALL involves complex combination of chemotherapy protocols, and hence individual SNPs might not have measurable effects on drug disposition and cure rates. Some adverse effects of ALL therapy have been linked to specific drugs. Candidate-gene and genome-wide approaches have identified inherited variants that may be associated with some of the risks of these drug-specific adverse effects in ALL.⁶¹

Earlier studies by Jamroziak et al⁶² and Stanulla et al⁶³ reported better EFS and a lower rate of CNS relapse associated with the ABCB1 T 3435 allele in childhood ALL. But Efferth et al⁶⁴ and Jamroziak et al⁶⁵ reported no association of this polymorphism with prognosis in adult ALL patients. Ceppi et al⁶⁶ observed that ABCB1 3435TT genotype had lower EFS in the discovery cohort in univariate and multivariate analysis. However, failure to subsequently replicate this finding in a validation patient set has raised arguments against the role of this polymorphism in modulation of ALL outcome. Erdélyi et al⁶⁷ examined the association of functional ABCB polymorphisms with acute side effects of chemotherapy in 138 Hungarian ALL children treated with the ALL-BFM-95 protocol. A higher proportion of patients who carried the ABCB1 3435TT genotype suffered excessive infectious complications than those harboring at least one C allele. These researchers concluded that ABCB1 3435T>C genotype was associated with the infectious complications of applied chemotherapy regimen.

However, single SNPs can have measurable effects if they either affect antileukemic agents such as 6-mercaptopurine or methotrexate (MTX) that are used extensively in the protocols,⁶⁸ or when the gene in question belongs to cytochrome P450 family,⁶⁹ or glutathione S-transferases,⁷⁰ and potentially the ABCB1 gene.

Although MTX is not considered as a P-gp substrate, studies of patients on MTX monotherapy by Grabar et al⁷¹ and Kato et al⁷² showed that the silent ABCB1 polymorphism 3435C>T might affect outcome and toxicity after MTX therapy. In a recent study on 522 Danish children with ALL, Gregers et al⁷³ explored the impact of ABCB1 polymorphisms 1199G>A, 1236C>T, 2677G>T/A, and 3435C>T on the risks of relapse and toxicity. They reported that the genetic variants 1199G>A and 3435C>T were associated with outcome in childhood ALL. In the high-risk patients who were carriers of 1199 GA variant, an overall relapse rate of >29% and a relapse rate of >60% were observed. Gregers et al⁷³ concluded that 1199G>A might be a new possible predictive marker for outcome in childhood ALL and that patients with 1199G>A polymorphism should be observed more intensively. In pediatric ALL patients from China, Liu et al⁷⁴ studied the association of 12 SNPs in 4 candidate genes of the MTX/folate pathway with pharmacokinetics, toxicity, and outcome. They reported that long-term outcome was better in ABCB1 rs1128503T and TC allele carriers than patients with C allele (92.7±1.6% vs 78.2±6.6%, p=0.020).

ABCB1 gene is involved in vincristine transport. A candidate gene study by Ceppi et al⁶⁶ demonstrated that variants in ABCB1 were associated with vincristine neurotoxicity during ALL therapy. The two ABCB1 variations (rs10264856 and rs4728709) were reported to be associated with increased risk of relapse in childhood ALL patients in a large genome-wide association study.⁷⁵ Because these two SNPs are in LD, (but not in LD with SNPs at 3435, 2677, or 1236 positions), Ceppi et al⁶⁶ analyzed one of these two SNPs (rs4728709). Although no association with EFS or OS was found, they⁶⁶ observed protective effect of rs4728709 against lower grades of neurotoxicity. From these results, Ceppi et al.⁶⁶ concluded that rs4728709 of ABCB1 (or other SNPs in LD) indeed has an impact on ALL treatment outcome, especially on vincristine-related neurotoxicity. Substitution (rs4728709) in the promoter of the ABCB1 was reported to have a protective effect against lower-grade neurotoxicity, and C to A variation (rs3770102) located 17 nucleotides upstream from transcription start site had a protective effect against high-grade neurotoxicity.⁶⁶ In another study by Zqheib et al⁷⁶ on 127 Lebanese ALL patients, a statistically significant association was found among neutropenia (absolute neutrophil count <500) and variant allele carriers of ABCB1 rs1045642 and ABCB1 rs1128503. According to these authors, genotyping for ABCB1 polymorphisms might be useful in identifying patients at risk of increased MTX toxicity, which warrants the need for dose optimization before treatment initiation. On the contrary, candidate gene studies by Kishi et al⁷⁷ and Guilhaumou et al⁷⁸ observed no association of ABCB1 variants and vincristine neurotoxicity.

Hence, it is probable that polymorphisms in ABCB1 gene may affect intrinsic blast resistance and pharmacokinetics of several drugs used in ALL protocols, thereby affecting the efficacy of treatment and EFS.

ABCB1 variants and CML treatment

Although targeted therapy with IM demonstrates high efficacy in most CML patients, nearly 35%–40% of CML patients on IM therapy develop resistance to IM. Resistance to IM could be due to a heterogeneous array of mechanisms involving BCR/ABL-dependent pathways and BCR/ABL-independent pathways.^78,79 BCR/ABL-dependent mechanism generally includes point mutations within the BCR/ABL kinase domain that interfere with IM binding and also overexpression or amplification of the BCR/ABL gene. Among BCR/ABL-independent mechanisms, a number of factors may influence the plasma and tissue levels of IM and, under certain circumstances, contribute to pharmacologic resistance. The efficacy and toxicity of IM seem to depend on both IM pharmacokinetics influenced by several enzymes and transporters, and IM pharmacodynamics influenced by mutational studies of the target. Recently, great attention has been focused on interpatient pharmacokinetic variability, which is due to patient’s inherent genetic constitution, as a BCR/ABL-independent mechanism mediating resistance to IM.

Drug exposure below the target level could lead to IM levels that are insufficient to inhibit BCR/ABL and to achieve optimal response. Of the varied reasons, aberrant expression of drug transporters also accounts for IM resistance. Polymorphisms in ABCB1 are likely to influence intracellular drug delivery, and therefore the effectiveness of IM which is a substrate of the P-gp-mediated efflux. Because of the same reason, ABCB1 SNPs could affect IM’s bioavailability and consequently the treatment outcome of IM therapy, which partially explains variable responses to IM.^80,81

SNPs in ABCB1 have been demonstrated to display high affinity for IM and confer IM resistance in vitro by extruding IM from hematopoietic cells.^82,83. ABCB1 polymorphisms were hypothesized to be functional polymorphisms altering mRNA stability, modifying the P-gp expression and therefore reducing IM substrate specificity.

In Indian CML patients, Sailaja et al⁸⁴ reported a higher frequency of 3435TT genotype in minor/major cytogenetic response (CyR) group compared to non-CyR group, but the overrepresentation of 3435TT genotype was statistically not significant. Dulucq et al⁸⁵ reported that the distribution of 3435C>T genotypes was not significantly associated with MMR (p=0.20) in Caucasian CML patients. Angelini et al⁸⁶ also recapitulated that the 3435CC genotype was significantly associated with complete molecular response among Caucasian CML patients. In this sense, the T allele has lower ABCB1 transcript levels compared to the C allele, and contributes to better IM response. On the contrary, a higher risk for IM resistance was reported for CML patients with homozygous T allele at 3435 locus.⁸⁷ Maffioli et al⁸⁸ also demonstrated inadequate response or failure to IM treatment associated with 3435TT genotype. Studies conducted on Malaysian CML patients had shown no relationship between 3435C>T polymorphism and response to a standard dose of IM.⁸⁹ In a recent study by Salimizand et al,⁹⁰ CML patients with C allele of ABCB1 C3435T had poor cytogenetic response and TT3435 ABCB1 diplotype was significantly associated with accelerated phase of CML. This study also indicated that CML patients with TT3435 ABCB1 might be having weaker response to IM therapy.

It has been reported⁹⁰ that the ABCB1 2677 G>T/A single nucleotide substitution strongly affects the secondary structure of ABCB1 mRNA. A decrease in P-gp expression could result in higher intracellular concentration of IM. Although Dulucq et al⁹¹ observed a higher frequency of MMR in patients with non-G genotypes at position 2677, they could not confirm these results in a larger patient cohort later,⁸⁵ The ABCB1 2677 variant was associated with MMR in Malaysian CML patients.⁸⁹ For ABCB1 2677 T/A polymorphism, a better complete cytogenetic response was observed for patients with variant TT/AT/AA genotypes compared to other genotype groups. Almost similar with the findings of Au et al,⁸⁹ a better CCyR rate was observed among patients with ABCB1 2677 GA/AT/AA genotype in the study by Ni et al.⁸⁷ This has been attributed to the fact that carriers of the 2677 variant genotype tend to have lower P-gp mRNA expression than those who had 2677 wild-type genotype⁹¹ Galimberti et al⁹² examined the role of ABCB1 SNPs with IM resistance by conducting a study which comprised of 33 CML patients treated with IM. This study showed that CML patients who did not achieve at least major cytogenetic remission had higher levels of ABCB1 expression.⁹² Elghannam et al⁹³ investigated the association of G2677T SNP with IM response in Egyptian CML patients. Multivariate analysis showed GT genotype to be an independent risk factor for resistance, while TT genotype was found to be a protective factor against resistance to IM. So, G2677T polymorphism might be useful in response prediction to therapy with IM in CML patients.

In Malaysian CML patients on IM therapy,⁸⁹ resistance was significantly higher among patients homozygous for the ABCB1 1236CC genotype, compared to patients with good IM response. The result from the study by Au et al⁸⁹ is in accordance with the findings of Deenik et al,⁹⁴ in which patients with homozygous ABCB1 1236TT showed a higher probability of obtaining MMR.

In contrast, in the study by Ni et al⁸⁷ on the impact of these SNPs on IM response, the number of T alleles at loci 1236 and 3435 were found to correlate with resistance. Resistance was higher in those CML patients who were homozygous for the 1236T allele, compared to patients with CT/CC genotype groups. With regard to 2677T/A polymorphism, a better complete cytogenetic remission was observed for patients with genotypes AG/AT/AA compared to TT/GT/GG genotypes. In the case of C3435T polymorphism, patients with 3435TT/CT genotypes showed a higher resistance compared with patients with CC genotype. On the contrary, in the study by Maffioli et al,⁸⁸ the CC genotype of C3435T was associated with primary failure, whereas T allele of G2677T/A seemed to protect from priamry failure.

In a meta-analysis by Zu et al,⁹⁵ a significant association between C1236T polymorphism and increasing risk of IM resistance in Asian CML patients was observed. However, they noted no significant association for G2677T or C3435T polymorphism in Asian populations as well as Caucasian CML populations. Zheng et al⁹⁶ conducted a meta-analysis that combined data from 12 reports and included 1,826 patients. This meta-analysis showed that the 2677G allele or 3435T allele predicted a worse response to Imatinib in CML patients, whereas 1236CC genotype was associated with better response in CML patients from the Asian region. These reports suggest the usefulness of these three SNPs of ABCB1 as predictive markers for the therapeutic use of IM in CML patients.

ABCB1 haplotypes and IM resistance

At haplotypic level, 3435C>T is in strong LD with 1236C>T and 2677G>T/A, forming two major haplotypes of 1236C/2677G/3435C and 1236T/2677T/3435T with abundant frequencies. ABCB1 1236T/2677T/3435T haplotype was correlated to higher IM pharmacokinetics trough levels in CML patients.^91,97 In haplotype analysis of these three SNPs of ABCB1 in Malaysian CML patients on IM therapy, Au et al⁸⁹ observed that the wild-type ABCB1 haplotype 1236C/2677G/3435C was associated with IM resistance, which is in agreement with a report by Dulucq et al⁹¹ in Caucasian population. On the other hand, Maffioli et al⁸⁸ found a correlation between 1236C/2677G/3435C haplotype and IM resistance. In yet another study,⁹⁸ none of the ABCB1 haplotypes had any major influence on the efficacy of IM in K562 cells. An explanation to these contradictory results could be that these three ethnicity-related SNPs may have different distribution of genotype and haplotype frequencies when examined in different populations.

Ali and Elsalakawy⁹⁹ genotyped the three SNPs (C1236T, G2677T and C3435T) in 100 Egyptian CML patients undergoing IM therapy. They found that the optimal response rate did not differ significantly between C1236T, G2677T, or C3435T genotypes. However, optimal response was significantly different among patients with the CGC, TTT, TGC, CGT, TGT, CTC, CTT, and TTC haplotypes. The 1236T/2677G/3435T haplotype was significantly associated with lower probability of achieving optimal response. According to these authors, ABCB1 SNPs haplotype analysis should also be taken into account in order to get clearer insights into who is likely to respond optimally to IM for identifying CML patients who may not respond optimally to standard dose IM therapy and potentially need an individualized therapeutic approach.

In another recent study, Eadie et al¹⁰⁰ investigated whether early increase in ABCB1 mRNA expression (fold change from diagnosis to day 22 of IM therapy) could predict patient response. Patients exhibiting a high fold rise were significantly less likely to achieve early molecular response, and major molecular response, even when switched to nilotinib therapy. According to Eadie et al,¹⁰⁰ an increase in levels of ABCB1 mRNA may serve as easily translatable early warning assay for loss of response/development of resistance to IM and could serve to identify poor responders who may benefit from the addition of ABCB1 inhibitor to their treatment regimen or from switching to alternate therapies. This study highlights the importance of drug efflux transporters and indicates that ABCB1 mRNA levels may provide a valuable prognostic biomarker.

The most likely explanation for the association of ABCB1 with IM resistance could be that ABCB1 acted as the transporter for IM. Overexpression of P-gp at the cell surface reduces intracellular IM concentrations and leads to ineffective levels of the IM upon reaching its target.¹⁰¹ Thus, increased ABCB1 levels would lead to reduced IM intracellular levels, impaired BCR-ABL inhibition, and ultimately resistance to IM treatment.

There is paucity of information available regarding the impact of these SNPs on other tyrosine kinase inhibitor drugs used in CML treatment. Dessily et al¹⁰² investigated the impact of expression of ABCB1 1236C>T, 2677G>T/A, and 3435C>T polymorphisms on the antiproliferative effects of imatinib, nilotinib, dasatinib, and ponatinib using K562 cell lines. They observed resistance of K 562_C-G-C to IM compared with K 562_C-G-T, K 562_C-T-T at clinically relevant concentrations. They demonstrated that the wild type protein (ABCB1_C-G-C) exported IM more efficiently and thus conferred higher resistance to IM compared to the variant protein (ABCB1_T-T-T). Consistently, in cells expressing the variant protein, IM intracellular concentrations were also significantly higher than in cells expressing wild-type protein. These results not only suggest that the variant haplotype decreases IM transport by ABCB1 but also provide an explanation for previous studies that associated the wild-type haplotype (CGC) to IM resistance.^89,91,95,96 In contrast with IM, these polymorphisms did not affect intracellular concentrations of nilotinib and also demonstrated limited influence on the antiproliferative effects of nilotinib, dasatinib, and ponatinib. These results suggest that the ABCB1 SNPs 1236C>T, 2677G>T/A, and 3435C>T significantly affect the antiproliferative activity and intracellular concentrations of IM, but not, or to a much lesser extent of nilotinib, dasatinib, and ponatinib.

In most of the studies, only the ABCB1 1236T>C and ABCB1 2677G>T/A genotypes and ABCB1 1236C/2677G/3435C haplotype were found to be significantly associated with IM response, whereas the other SNPs did not show any significant association. This could be attributed to several factors. ABC transporters are subjected to drug–drug interactions and to regulation by intracellular receptors, cytokines, and epigenetic factors. It has to be noted that overexpression of ABCB1 is partly mediated by nuclear receptors like the pregnane X receptor. Also, suppression of micro RNAs also have been shown to lead to an upregulation of ABCB1.

ABCB1 polymorphisms and CLL treatment

For CLL, there are many current first-line treatment options. The choice of treatment depends on the stage of the disease, the patient’s symptoms, the age and overall health of the patient, and the benefits vs side effects of treatment. In a study on CLL patients, Jamroziak et al¹⁰³ found highest P-gp activity in the carriers of the 3435CC genotype followed by intermediate activity in 3435CT heterozygous subjects and the lowest activity in the carriers of 3435TT genotype. From the above findings, these authors concluded that genotype-related differences in P-gp activity in B-CLL tumor cells may have implications for response to chemotherapy with P-gp transported anticancer agents.

Dong et al¹⁰⁴ did not find any association of SNPs C1236T, G2677T/A, and C3435T of ABCB1 with clinical prognostic factors in Chinese CLL patients. Penna et al¹⁰⁵ evaluated whether the SNPs G2677T and C3435T provided any prognostic information on the clinical progression of B-CLL. The G2677T SNP was associated with the prognostic patients’ characteristics and poor prognosis, whereas C3435T showed no association with CLL prognosis. According to Penna et al,¹⁰⁵ ABCB1 heterozygosis may lead to a different functional capacity of the encoded protein and to a different mRNA expression with respect to homozygous state. Moreover, mutant heterozygous G2677T genotype could be clustered nonrandomly and nonuniformly (LD) with other genes that are able to induce a worse prognosis. These findings support the importance of considering ABCB1 polymorphisms as prognostic markers in patients with B-CLL in defining a more individualized prognosis and helping to identify patients who are at risk of rapid progression.

The conflicting findings reported in the literature could be attributed to several factors. Difference in demographic data from subjects selected for the various ABCB1 SNPs, especially difference in genetic background of the study subjects across the populations worldwide, could be an important factor. Likewise, difference in study design and sample size among the studies and different genotyping methods employed across studies also might be contributing. Moreover, strong LD between the SNPs and different unobserved causal SNPs in different study populations also may provide a plausible explanation for conflicting reports on association of the SNPs studied with IM response. According to Marchetti et al,¹⁰⁶ the methods used to measure P-gp expression in various studies, route of drug administration and extent of metabolism relative to P-gp-mediated transport, environmental factors including difference in dietary constituents among different populations that influence transporter function, involvement of other transporters, and associated genetic variability also contribute to the inconsistency in the reports worldwide. Noninclusion of haplotypes in several studies is another major factor of concern for contradictory reports. Therefore, more in-depth studies and increasing knowledge on function, regulation, and genetic variation of transporters are warranted, which can contribute to a better understanding on divergent results obtained.

Conclusion and future implications

There is accumulating evidence that treatment outcome in leukemias can be influenced by germ line polymorphisms that affect drug disposition and/or pharmacodynamics and that these effects may explain some of the variability in treatment outcome that cannot be explained by the genotype and phenotype variation in leukemic clone. Therefore, genetic variation in ABCB1 is of tremendous clinical interest in the pharmacokinetics of commonly used antileukemic drugs and in multidrug resistance.

The World Health Organization (WHO) classification¹⁰⁷ has included an increasing amount of new clinically relevant genomic information for the implementation of precision medicine programs. Few polymorphisms and haplotypes of ABCB1 have been associated with alterations in drug disposition and drug response, including adverse events with various ABCB1 substrates in different ethnic populations. These SNPs account for the interindividual differences in pharmacokinetics and clinical response of selected anti-leukemic drugs. But the data yielded are not in distinct and unconfined reproducible outcomes and are not yet conclusive enough to translate pharmacogenetic tests to clinical practice. In this context, ABCB1 transporter polymorphisms are not yet suitable to be used as biomarkers to predict therapeutic response in leukemias and so have not been included in the WHO classification. New biomarkers and pharmacogenetic tests are emerging only, and based on these, novel treatment protocols that are personalized to the genotype needs to be designed.

For a complete understanding of the contribution of genetic variability in ABCB1 and treatment response and toxicity in leukemias, additional studies involving larger sample sizes and stratification according to haplotype need to be carried out. Because of the known interpopulation differences in drug response, factors such as variability among ethnic groups, characterization of variability in haplotype structure, LD and recombination within and among ethnic populations, etc, should also be considered. It would also be ideal to carefully consider uniformity in demographic data of the subjects selected, sample size, environmental factors, and standardization of assays relating to ABCB1 mRNA and protein detection and quantification. Hence, future research activities, considering novel genomic methodologies such as deep sequencing approaches (next-generation sequencing),¹⁰⁸ and a comprehensive definition of clinical phenotypes based on a representative and valid sample size calculation,^19,109 to elucidate the impact of rare ABCB1 variants and their potential consequences for effect sizes are warranted. Individualized approaches based on pharmacogenomics profile of individual patients may offer more efficient and less toxic therapy to leukemias in the future and can lead to personalized approaches to diagnose and treat patients.

Disclosure

The author reports no conflicts of interest in this work.